This invention relates to a non-aqueous lithium ion secondary battery having a high discharge capacity.
As a non-aqueous lithium ion secondary battery for general use, a xe2x80x9crocking chair typexe2x80x9d battery (which comprises a combination of lithium ion receiving/releasing materials) is widely employed in these days. The battery generally comprises a negative electrode of one or more of various carbonaceous materials (namely, negative electrode active material) into which lithium ions can be reversibly intercalated, and a positive electrode of lithium-containing metal complex oxide (namely, positive electrode active material) which can reversibly receive and release lithium ions.
As the positive electrode active materials, LiCoO2, LiCo1xe2x88x92xNixO2, LiNiO2 and LiMn2O4 are generally employed. LiCoO2 is advantageously used because it, in combination with Li, gives a high voltage of more than 3.5 volts, and a high discharge capacity (shown in Japanese Patent Provisional Publication No. 55-136131). Japanese Patent Provisional Publications H3-147276 and H4-123769 disclose a secondary battery comprising a positive electrode active material of LiMn2O4. Since manganese is less expensive and more easily available than cobalt, the disclosed battery can be produced at a relatively low cost.
As the carbonaceous material for negative electrode active materials, graphite material, pitch coke, fibrous carbon, and soft carbon obtained by low temperature firing are known. However, since the carbonaceous material generally has a low bulk density of less than 2.20, the resultant battery shows a low discharge capacity even if the material is made to retain lithium ions at the stoichiometric maximum of intercalating capacity (372 mAh/g). In order to solve the problem, some publications propose negative electrode active materials of amorphous metal complex oxide mainly comprising tin oxide (Japanese Patent Provisional Publications No. H6-60867, No. H7-220721, No. H7-122274 and No. H7-288123, and PCT WO96-33519).
The above-mentioned amorphous metal complex oxide (negative electrode active material) in combination with cobalt oxide (positive electrode active material) gives a battery of a high energy density, but thus composed battery is considerably expensive. On the other hand, a battery comprising a negative electrode of the amorphous metal complex oxide and a positive electrode of manganese oxides has good cost performance, but its energy density is considerably small.
A battery comprising a positive electrode of nickel oxide has both good cost performance and a high discharge capacity. However, the battery employing LiNiO2 (basic component of the positive electrode of nickel oxides) generally has such drawbacks that the average discharge voltage is lower than that of the battery using LiCoO2 by 0.2 volt or more, and that its cycle capacity retention is relatively low. Because of the low discharge voltage, LiNiO2 cannot effectively work in the low voltage range so that the capacity of the battery is restricted to an unsatisfactory level. Therefore, in order to improve the properties of positive electrode nickel oxide having LiNiO2 skeleton, its components must be studied.
In a high capacity battery, the active materials of positive and negative electrodes generally have large Li-intercalating/releasing capacities per volume, and accordingly they have large volume expansion-shrinkage coefficients. Therefore, since the active materials likely vary their volume to a large extent, the battery is liable to exhibit unstable performance (e.g., deterioration of cycle capacity retention).
Accordingly, it is an object of the present invention to provide a non-aqueous lithium ion secondary battery having a high discharge capacity and exhibiting reliable performance.
The invention resides in a non-aqueous lithium ion secondary battery which comprises a positive electrode layer comprising an active material of lithium-containing metal complex oxide, a negative electrode layer comprising lithium ion retainable and releasable material, and a non-aqueous electrolyte, wherein
the lithium-containing metal complex oxide is a nickel-containing lithium complex oxide of the following formula:
LixNi1xe2x88x92yCoyxe2x88x92zMzO2xe2x88x92aXb
wherein M represents at least one element selected from the group consisting of the elements belonging to the groups 2 to 14 of the periodic table, particularly, Ca, Ba, Ti, V, Cr, Mn, Fe, Co, Cu, Zn, Nb, Mo, Ag, Ta, W, B, Al, Ga, In, C, Si, Ge, Sn and Pb; X is a halogen atom; x, y, z, a and b are numbers satisfying the conditions of 0.2 less than xxe2x89xa61.2, 0 less than yxe2x89xa60.5, z less than y, 0 less than z less than 0.5, 0xe2x89xa6axe2x89xa61.0 and 0xe2x89xa6bxe2x89xa62a, respectively; and
at least one of positive and negative electrode layers contains rubber binder.